1. Field of the Invention
The present invention relates to a theft monitoring tag for informing that an article is being stolen.
2. Description of the Related Art
A theftproof tag is conventionally disclosed in which a resonance circuit of the tag attached to an article resonates with an electric wave at a specified frequency output from a wave transmitter, a separation detecting means detects whether or not the theft monitoring tag is separated from the article, and a separation informing unit controls a sound output on the basis of the detection output of the separation detecting means (Japanese Unexamined Patent Publication No. 8-185584). In this theftproof tag, the resonance circuit comprises an insulating dielectric thin film and a conductive metallic foil formed in a predetermined shape on either side of the thin film by etching or the like. For example, a coil unit is formed in a spiral form on the surface of the thin film by using a conductive metallic foil, and a surface-side plane pattern of a capacitor connected to the coil unit is formed at the center of the spiral shape of the coil unit.
A transmitting antenna and a receiving antenna are provided in a standing condition at the entrance of a store which sells the theft monitored article at a predetermined distance therebetween, with these antennas being electrically connected to a control unit. The control unit controls the transmitting antenna to transmit an electric wave at a frequency at which the resonance circuit resonates, and checks the signal level of the received signal of the receiving antenna. A speaker is connected to the control output from the control unit for generating an alarm.
In the theftproof tag constructed as described above, when a theft monitored article passes between the transmitting and receiving antennas without payment of money, the resonance circuit of the tag attached to the theft monitored article resonates with an electric wave transmitted from the transmitting antenna, and the receiving antenna receives a received signal modulated to the receiving level. As a result, the control unit controls the speaker to generate an alarm, thereby preventing the article from being stolen without payment of money. When money is paid for the article, a store clerk applies a strong electromagnetic wave to the tag to break the capacitor so that the tag does not operate, or temporarily stops the alarm speaker so as not to generate an alarm.
However, in the conventional theftproof tag, the center line of the spiral coil unit extends perpendicularly to the attaching surface of the article, and thus the electric wave transmitted from the resonance circuit passes through the article. Therefore, if the tag is attached to an article with the surface made of a conductive material such as aluminum, or a ferromagnetic material such as a steel sheet, the magnetic flux generated in the resonance circuit passes through the article to change the self-inductance of the coil unit. Thus, the resonance frequency of the resonance circuit is changed to decrease the Q value, thereby causing the possibility that the tag will not operate, as compared with a tag attached to an article with the surface made of an insulating material or a nonmagnetic material. Assuming that the angular frequency is L, and the resistance component of the resonance circuit is r, the Q value is defined as L/r. It is known that as the Q value increases, the loss due to an eddy current or the like decreases, and the resonance width decreases.